1. Field of the Invention
The present invention relates to a seal structure for a steam turbine.
2. Description of the Related Art
A power plant generates electricity by rotating a turbine (steam turbine) by steam which a steam generator such as a boiler generates. As steam turbines used in such a power plant, a high-pressure turbine, an intermediate-pressure turbine and a low-pressure turbine are provided in order from the upstream side of flow of the steam. The steam used for rotating the low-pressure turbine is introduced into a condenser through an exhaust hood. The steam is then condensed by the condenser and turned into water. The water is returned to the steam generator.
In the steam turbine constituting a part of the power plant, each of stator blades fixed to the inner side of a casing is arranged between rotor blades that rotate with a rotor. The rotor blade and the stator blade form a stage.
The steam introduced into the casing flows in the casing of the steam turbine. The steam then alternately passes the stator blades and the rotor blades (that are fixed to the rotor rotatably held by the casing) and expands so as to rotate the rotor. After passing through the rotor blade provided on the most downstream side of the rotor, i.e., the last-stage rotor blade, the steam is discharged outside of the casing.
In the steam turbine, the steam causes the rotor blades to rotate and thereby causes the rotor to rotate. In order to efficiently utilize the steam, it is required to minimize a clearance between a fixed portion and a rotating portion, such as clearances between the stator blades and the rotor, in order to reduce the amount of steam leaking from the clearance as much as possible and to improve a sealing property. However, when the clearance between the rotating portion and the fixed portion is minimized, the frequency of contacts between the rotating portion and the fixed portion increases. Frictional heat generated by the contacts may increase the temperature of a contact part of the rotating portion. When the contact part of the rotation portion increases in temperature due to the frictional heat, a temperature distribution of the rotating portion (in a rotational circumferential direction of the rotating portion) is not uniform, so that a rotor shaft may be bent by heat or the rotating portion may vibrate. When the amplitude of the vibrating motion is large, the steam turbine needs to be stopped in some cases.
In order to cope with the abovementioned problems, a technique relating to a seal structure is disclosed (for example, refer to JP-A-2002-228013 (refer to FIG. 1)). In the technique described in JP-A-2002-228013, a labyrinth seal device having a fin (seal fin) is conventionally provided between a rotating portion such as a rotor and a fixed portion such as a stator blade; and a member (abradable member) that has an excellent cutting property and faces the fin is used. According to the technique described in JP-A-2002-228013, in the case where the labyrinth seal device is designed to ensure that a clearance between the fin and the abradable member is minimized, when the fin and the abradable member contact each other for some reason, the abradable member is cut by the fin. This can suppress generation of frictional heat due to the contact of the fin with the abradable member. In addition, it is possible to suppress a vibrating motion of the rotor shaft due to nonuniform heat bending (occurring due to an increase in the temperature of the rotor) of the rotor shaft.
JP-A-2007-16704 discloses a technique relating to a seal structure having a heat insulating layer for suppressing transfer of heat to a rotating portion. According to the technique described in JP-A-2007-16704, the heat insulating layer is located between the rotating portion and a fin provided at the rotating portion, for example. This arrangement suppresses transfer of frictional heat (to the rotating portion) generated by a contact of the rotating portion with a fixed portion and suppresses a vibrating motion of a rotor shaft due to nonuniform heat bending of the rotor shaft.